Breast cancer is a devastating disease responsible for the second highest number of cancer-related deaths in women in the USA (ACS, 1992). Over one in ten women will develop breast cancer in their lifetime, and greater than 30% of cases die from the disease every year. Patient management is often difficult because clinical progression varies considerably from patient to patient. There is therefore an urgent need to develop molecular markers that will aid in the understanding of the events in breast cancer development. We have functionally identified a novel breast cancer growth suppressor gene on chromosome 17 using microcell-mediated chromosome transfer (MMCT). The transfer of a normal neomycin-tagged chromosome 17 results in the growth suppression of the MCF 7 breast cancer cell line in vitro after 10-12 population doubling. In contrast, a control chromosome 13 had no effect upon growth of these cells either in vivo or in vitro. Furthermore we have shown that the p53 tumor suppressor gene is not responsible for this phenotype. These data strongly support the hypothesis that there is a gene on chromosome 17 capable of the growth suppression of breast cancer cells. This novel growth suppressor gene potentially represents a critical gene in breast cancer development, as rearrangements or deletions of chromosome 17 are the most frequently observed events in breast cancer. Using the in vitro growth assay that we have developed we proposed the following two specific aims: 1. To identify the MCF 7 breast cancer growth suppressor gene. Using the following approaches: (i) Define the region on chromosome 17 by MMCT of chromosome 17s containing discrete deletions, and by LOH studies of primary breast tumors; (ii) Identify chromosome region-specific YACs, cosmids and phage. This will be achieved by generating region-specific IRS-PCR probes using a radiation-reduced hybrid panel of chromosome 17; (iii) Identify genomic rearrangements or deletions in breast cancer cell lines and primary breast tumors using region-specific clones. 2. To identify candidate genes from region-specific clones and to functionally test their effect upon growth suppression in MCF 7 cells by DNA transfection. Using the following approaches: (i) Identify expressed sequences from region-specific cosmid and phage clones using exon amplification; (ii) Isolated full length clones by RACE and cDNA library screening; (iii) Directly assay for growth suppression using region-specific YACs into MCF 7 cells; (iv) Provide functional evidence for the cloning of the novel growth suppressor gene by DNA transfection into MCF 7 cells. The long term goal of this proposal is to identify the novel chromosome 17 growth suppressor gene, and to determine its significance in primary breast cancer. We believe that the cloning and analysis of this gene will prove to be a great benefit both in the diagnosis and ultimately the treatment of primary breast cancer.